The present invention is related to measurement methods wherein a dimensional entity of a piece to be checked is measured, and the measured value is then compared with the value of a corresponding dimensional entity of a reference sample, so as to detect a deviation value, if any.
In metrological applications there is the need of providing systems which may improve the resolution and the accuracy of the measurements. A further relevant requirement is the capability of reproducing a measurement: this enables, for a given reference sample, to read again, under determined physical and environmental outline conditions, the characteristics of the sample itself with a known uncertainty.
Thus, for example, between the institutes which are part of B.I.P.M. (Bureau International des Poids et Misures) and the qualified laboratories, "circulating samples" are circulated, the aim of which is to evaluate the capability of the laboratories to reproduce the peculiar magnitudes of the sample.
In general the total uncertainty in the measurement of a sample is dependent upon the contribution of each component intervening in the measurement process, in accordance with known relationships.
For sure a fundamental component intervening in the measurement process is the temperature. Variations of one degree of temperature on a sample having a length of 100 mm, made of steel, lead to an elongation of 0.00115 mm. These variations occur according to typical paths of the heat transmission with various time constants, depending upon the physical-environmental characteristics of the laboratory.
Extending the latter considerations to different environments other than a laboratory, such as for instance a workshop within the frame of product quality testing; it can be realised how critical is the temperature factor wherever the variations thereof are well over one degree.
It is known that percentage tests, which are meaningful in connection with workpieces, are carried out within production environments, either manually or automatically.
It is also known that these tests are carried through on related instruments comparing the measurement made on the piece with that cyclically made on comparison samples which the instruments are equipped with. These samples, defined also as "zero-setting samples", are intended to define, accomplish, store or reproduce one or more known value in order to transmit same by comparison to others. Now, since the zero-setting sample is certified under a temperature of 20.degree. C. on nominal values, according to the traditional dimensional measurement methods the relative measurements are compared under the hypothesis that the pieces which are subjected to testing and the zero-setting sample have the same thermal expansion coefficient.
EP-A-0498780 discloses a method and arrangement for determining and compensating for linear heat expansion of elongated bodies, such as physical length scales and the like in instruments and measuring machines in which measurement errors occur because of length deviations caused by deviations from a reference temperature at which the length of the body was calibrated. This method and arrangement are also based upon measuring the linear expansion of the body in relation to a reference made of a material whose coefficient of linear heat expansion deviates from the coefficient of linear heat expansion of the body material, and thereafter determining, when appropriate, a compensation factor for correcting the body length to be determined.
This is what is presently provided for in the state of the prior art of workshop metrology. The apparatus employed therefor are normally based upon computerised systems, the software of which may also contemplate implementations which take into account the temperature trend. However, these expedients are in general not completely reliable, and due to this reason in practice, some are not suitable for use in workshop environments.